1. Field of the Invention
The present invention relates to a piezoelectric resonator and a piezoelectric resonance apparatus for use in resonators, oscillators, or other electronic devices, and more particularly, to a thickness extensional vibration piezoelectric resonator and a piezoelectric resonance apparatus each operative to utilize higher harmonics in a thickness extensional vibration mode.
2. Description of the Related Art
Piezoelectric resonators are used in a variety of piezoelectric resonance apparatuses such as piezoelectric oscillators, piezoelectric filters, and so forth. Conventional piezoelectric resonators of this type are operative to utilize different piezoelectric vibration modes depending on the frequencies used.
Japanese Unexamined Patent Publication No. 1-117409 discloses an energy-trapping piezoelectric resonator operative to utilize the second harmonic in a thickness extensional vibration mode. This piezoelectric resonator will be described with reference to FIGS. 19 and 20.
The above-mentioned piezoelectric resonator is formed by laminating ceramic green sheets 61 and 62 made of a piezoelectric material, and firing the green sheets 61, 62 integrally, as shown in the exploded perspective view of FIG. 19. A circular excitation electrode 63 is located on the ceramic green sheet 61 at the center thereof. The excitation electrode 63 extends to one of the side edges of the ceramic green sheet 61 through an outgoing electrode 64. Further, a circular excitation electrode 65 is provided at the center of the upper side of the ceramic green sheet 62. The excitation electrode 65 extends to one of the side edges of the ceramic green sheet 62 through an outgoing electrode 66. Moreover, on the underside of the ceramic green sheet 62, an excitation electrode 67 is provided and extends to one of the side edges of the ceramic green sheet 62 through an outgoing electrode 68, as shown in the downward projection.
The above-mentioned ceramic green sheets 61 and 62 are laminated, pressed in the thickness direction, and then fired. The obtained sintered material is polarized so that a piezoelectric resonator 70 as shown in FIG. 20 is produced.
In the piezoelectric resonator 70, piezoelectric layers 71 and 72 defined by the sintered materials, are polarized evenly in the thickness direction indicated by the arrows in FIG. 23.
In operation, the piezoelectric resonator 70 is resonated by connecting in common the excitation electrodes 63 and 67 to apply an AC voltage between the excitation electrodes 63 and 67 and the excitation electrode 65. In this case, the vibration energy is trapped in the area where the excitation electrodes 63, 65, and 67 are overlapped, that is, a resonance portion A.
The conventional piezoelectric resonator 70 is operative to utilize higher harmonics in a thickness extensional vibration mode and is constructed to define an energy-trapping piezoelectric resonator, as described above. Accordingly, it is necessary to provide a vibration-attenuating portion on the periphery of the resonance portion A for attenuation of the vibration. That is, it is required to provide the vibration attenuating portion of which the area is larger than that of the resonance portion A. Therefore, it has been difficult to miniaturize the piezoelectric resonator 70.
On the other hand, Japanese Unexamined Patent Publication No. 2-235422 discloses an energy-trapping piezoelectric resonator containing a strip-type piezoelectric ceramic, in which it is not necessary to provide an piezoelectric substrate portion with a large area on the periphery of its resonance portion.
In the energy-trapping type piezoelectric resonator, an excitation electrode 82a is provided on the upper side of an elongated piezoelectric substrate 81, and an excitation electrode 82b is provided on the underside thereof, as shown in FIG. 21. Each of the excitation electrodes 82a and 82b is extended to a pair of the longer sides of the piezoelectric substrate 81 and extends over an entire width thereof. Further, the back side of the excitation electrode 82a and the front side of the excitation electrode 82b are opposed to each other in the center in the longitudinal direction of the piezoelectric substrate 81 so as to define a resonance portion. Further, the excitation electrodes 82a and 82b are extended to the ends 81a and 81b in the longitudinal direction of the piezoelectric substrate 81.
In the piezoelectric resonator 80, when the thickness extensional vibration mode is excited, unnecessary vibrations are generated, based on the dimensional relationship between the width W and the thickness T of the piezoelectric substrate 81. In Japanese Unexamined Patent Publication No. 2-235422, it is described that when the fundamental wave is utilized, the ratio of W/T of about 5.33 at a resonance frequency of 16 MHz is suitable, and when the third harmonic is utilized, unnecessary spurious components between the resonance frequency and the anti-resonance frequency can be reduced by setting the ratio of W/T at about 2.87 when the resonance frequency is about 16 MHz.
As described above, in the case of the energy-trapping piezoelectric resonator operative to utilize the second harmonic in a thickness extensional vibration mode, disclosed in Japanese Unexamined Patent Publication No. 1-117409, it is necessary to provide a large vibration attenuating portion on the periphery of its resonance portion. Accordingly, miniaturization of the energy-trapping type piezoelectric resonator is difficult.
Moreover, referring to the energy-trapping type piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 2-235422, it is unnecessary to provide a vibration-attenuating portion on the side of the resonance portion, and therefore, miniaturization can be achieved. However, when higher harmonics in a thickness extensional vibration mode are practically utilized, spurious components appear between the resonance and anti-resonance frequencies. Accordingly, in some cases, effective resonance characteristics can not be obtained.
Referring to the piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 2-235422, the electric capacity is relatively small so that the piezoelectric resonator is easily affected by a floating capacity generated from the circuit board or other components.